You can change it yourself, if you installed the theme manually goto~/.themes/Orta/gtk-2.0/
and if you have installed it via ppa goto/usr/share/themes/Orta/gtk-2.0/
and open the gtkrc file you will find there. At line 13 you will find this :gtk-icon-sizes = "panel-menu=22,22:gtk-button=16,16"
change it to this :gtk-icon-sizes = "panel-menu=16,16:gtk-button=16,16"
save the file and reload the theme.